Process for decomposing fluorine compounds

ABSTRACT

A fluorine compound having iodine within the molecule is decomposed by a decomposition reactive agent comprising alumina and an alkaline earth metal compound, the produced chlorine, fluorine and/or sulfur are fixed as a chloride, a fluoride and/or a sulfate of alkaline earth metal in the reactive agent, and iodine, which cannot be fixed as a salt of alkaline earth metal, is removed by an adsorbent.

CROSS-REFERENCE TO RELATED APPLICATION

This application is an application filed under 35 U.S.C. §111(a)claiming benefit pursuant to 35 U.S.C. §119(e)(1) of the filing date ofthe Provisional Application 60/447,004 filed Feb. 13, 2003, pursuant to35 U.S.C. §111(b).

TECHNICAL FIELD

The present invention relates to a process for decomposing fluorinecompounds. More specifically, the present invention relates to a processfor decomposing fluorine compounds, having iodine within the molecule,to render them harmless.

BACKGROUND ART

Heretofore, so-called PFC (perfluorocarbon) gases such as CF₄, C₂F₆ andC₄F₈ have been mainly used in the etching or cleaning performed by usinga silicon-based compound. However, these gases have a problem in thattheir lifetime in air is long and their global warming coefficient ishigh. From the standpoint of preventing global warming, a greatreduction in their use has been mandated by the Kyoto Protocol (COP3).To satisfy this requirement, the development of etching or cleaningmaterials having a short lifetime in air and a low global warmingcoefficient is proceeding.

On the other hand, the integration degree of semiconductors isincreasing at a rate of nearly 1.5 times per year and the precision inthe processing technique therefor is also becoming higher. Recently, thedevelopment of etching materials or semiconductor production devicescapable of nano-order fine processing has also proceeded.

In recent years, fluorine compounds having iodine within the molecule,such as CF₃I, are attracting attention because the iodine-containingfluorine compounds are expected to enable fine processing and are low inthe global warming effect.

Furthermore, in order to reduce emission of fluorine compound gaseshaving a high global warming coefficient (for example, CF₄ and C₂F₆),many devices for decomposing a fluorine compound gas after use andrendering it harmless are being studied. For example, (1) a burning anddecomposition method of treating the gas together with fuel (see,WO94/05399), (2) a thermal decomposition method of using a reactiveagent such as silica and zeolite (see, Japanese Unexamined PatentPublication No. 7-116466 (JP-A-7-116466)) and (3) a catalyticdecomposition method of using alumina or the like (see, JapaneseUnexamined Patent Publication No. 10-286434 (JP-A-10-286434)) are known.

However, the gas to be treated by these decomposition devices is afluorine compound containing carbon, fluorine, chlorine or hydrogen, andthe devices are not considered suitable to decompose a fluorine compoundhaving iodine within the molecule and render it harmless. This isbecause the iodine-containing fluorine compound may be decomposed by anymethod but iodine is contained in the gas after decomposition and mustbe separately removed.

In the case where a fluorine compound having iodine within the moleculeis used for etching, a difficult-to-decompose fluorine compound having ahigh global warming coefficient is also produced as a by-product in theform of a perfluorocarbon such as CF₄ or the decomposed iodine is alsocontained in the gas and therefore, these compounds must be alsodecomposed and/or rendered harmless at the same time.

Iodine is contained in gargles or the like and is known to have anantiseptic effect. However, it is also known that an excess ingestion ofiodine causes thyropathy. Therefore, emission of untreated iodine, as itis, into the environment is not preferred.

Furthermore, iodine as a by-product produced by the use of a fluorinecompound having iodine within the molecule in an etching device oriodine as a by-product resulting from decomposition of the fluorinecompound by a decomposing device is in a gaseous state. The acceptableconcentration of gaseous iodine is as low as 0.1 ppm and the contact ofthis gas with a human body is dangerous. Therefore, the amount ofgaseous iodine must be reduced to a harmless level.

DISCLOSURE OF INVENTION

Under these circumstances, an object of the present invention is toprovide a process capable of decomposing a fluorine compound havingiodine within the molecule or a compound contained in an exhaust gasgenerated on use of this fluorine compound in etching, rendering thecompound harmless by removing decomposed products, and realizing safedischarge into the environment.

As a result of extensive investigations to attain the above-describedobject, the present inventors have found that a fluorine compound havingiodine within the molecule can be rendered harmless by decomposing itusing a decomposition reactive agent containing alumina and an alkalineearth metal compound preferably at a temperature of 200° C. or more,fixing the produced chlorine, fluorine and/or sulfur as a chloride, afluoride and/or a sulfate of an alkaline earth metal in the reactiveagent, and adsorbing iodine which cannot be fixed as a salt of analkaline earth metal, to an adsorbent and, if desired, when a net-likesubstance for fixing iodine of vapor pressure or more is provided at theinlet portion of adsorbent layer, the pathway can be prevented fromclogging due to solidification of iodine of vapor pressure or more atthe adsorbent layer inlet. The present invention has been accomplishedbased on this finding.

Accordingly, the present invention relates to processes for decomposingfluorine compounds described in [1] to [15] below.

[1] A process for decomposing a fluorine compound, comprising bringing agas containing a fluorine compound having iodine within the moleculeinto contact with a reactive agent containing alumina and an alkalineearth metal compound, and then bringing the gas, obtained after thecontact, into contact with an adsorbent.

[2] The process as described in [1] above, wherein the alumina ispseudo-boehmite alumina.

[3] The process as described in [1] or [2] above, wherein the alkalineearth metal compound is a carbonate of magnesium, calcium, strontiumand/or barium.

[4] The process as described in any one of [1] to [3] above, wherein thereactive agent contains an oxide of at least one metal selected from thegroup consisting of copper, tin, nickel, cobalt, chromium, molybdenum,tungsten and vanadium.

[5] The process as described in any one of [1] to [4] above, wherein thealkali metal content of the reactive agent is 0.1 mass % or less.

[6] The process as described in any one of [1] to [5] above, wherein thereactive agent is a granule having a particle size of 0.5 to 10 mm.

[7] The process as described in [6] above, wherein the water content ofthe reactive agent is 1 mass % or less.

[8] The process as described in any one of [1] to [7] above, wherein thefluorine compound having iodine within the molecule is contacted withthe reactive agent at a temperature of 200° C. or more.

[9] The process as described in any one of [1] to [8] above, wherein thegas containing a fluorine compound having iodine within the molecule isan exhaust gas generated during use of the fluorine compound for etchingor cleaning.

[10] The process as described in any one of [1] to [9] above, whereinthe gas containing a fluorine compound having iodine within the moleculeis contacted with the reactive agent at a temperature of 500° C. or morein the presence of oxygen to thereby restrain the production of carbonmonoxide.

[11] The process as described in any one of [1] to [10] above, whereinthe adsorbent is at least one adsorbent selected from the groupconsisting of activated carbon, alumina, silica gel and zeolite.

[12] The process as described in any one of [1] to [11] above, whereinthe gas is contacted with the adsorbent at a linear velocity of 20Nm/min or less.

[13] The process as described in any one of [1] to [12] above, wherein alayer for fixing iodine is provided in front of the layer packed withthe adsorbent.

[14] The process as described in [13] above, wherein the layer forfixing iodine has a porosity of 80% or more.

[15] The process as described in any one of [1] to [14] above, whereinthe fluorine compound having iodine within the molecule is at least onecompound selected from the group consisting of fluoroiodocarbon,hydrofluoroiodocarbon, chlorofluoroiodocarbon andhydrochlorofluoroiodocarbon.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an equipment arrangement diagram showing one example of theapparatus for practicing the process of the present invention.

BEST MODE FOR CARRYING OUT THE INVENTION

The preferred embodiment of the present invention is described in detailbelow.

Examples of the fluorine compound having iodine within the molecule,which can be decomposed and rendered harmless by the process of thepresent invention, include compounds such as CF₃I, CF₂I₂, CFI₃, CHF₂I,CH₂FI, CClF₂I, CClFI₂, CHFI₂, C₂F₅I, C₂F₄I₂, C₂ClF₄I, C₂ClF₃I₂,C₂Cl₂F₂I₂, C₂HF₄I, C₂HF₃I₂, C₂H₂F₃I, C₂HClF₃I, C₂F₃I, C₂F₂I₂I, C₂HF₂I,C₂F₅IO and C₂F₄I₂O.

In decomposing such a fluorine compound having iodine within themolecule according to the process of the present invention, the gas ofthis compound may be diluted with an inert gas such as helium, argon andnitrogen or with air, or the gas may be a mixed gas which is liquid atordinary temperature but, when accompanied with other inert gas or air,comes to contain the vapor thereof in an amount of 0.01 vol % or more.The gas may be a single gas or may be a mixture of two or more gases.

According to the present invention, for example, a hard-to-decomposecompound having a high global warming coefficient, such asperfluorocarbon (e.g., CF₄, C₂F₆) produced as a by-product during use inetching or the like, and a compound such as HF, SiF₄ and COF₂, can bealso rendered harmless by decomposing such a compound in the same mannerand fixing it as an alkaline earth metal fluoride (for example, CaF₂).Furthermore, iodine which is difficult to fix as an alkaline earth metalsalt can be rendered harmless by removing it using an adsorbent.

Here, the reason why the iodine component after decomposition cannot befixed as an alkaline earth metal salt unlike the fluorine component isbecause the alkaline earth metal iodide is unstable as compared withalkaline earth metal fluoride and cannot be stably present under thetemperature condition of the thermal decomposition.

Therefore, iodine incapable of being fixed as alkaline earth metaliodide comes out from the outlet of a decomposition reactor where theiodine-containing fluorine compound is decomposed by a decompositionreactive agent, and this iodine must be removed from the standpoint ofensuring safety of the environment and of living things.

To cope with this, in the present invention, an adsorbent for removingiodine is provided in the post-stage of decomposition reactor, wherebyiodine, which is to fix on a reactive agent, is removed and renderedharmless.

The adsorbent for removing iodine may be a generally availablecommercial product such as active carbon, alumina, silica gel andzeolite, or a product enhanced in the adsorbing ability by performing aspecial treatment such as attachment of a metal or the like on thecommercially available product. In particular, active carbon ispreferred because it exhibits a high adsorbing ability in the removal ofiodine and is inexpensive. However, activated carbon may burn under thecondition of, for example, high oxygen concentration by the effect ofheat generation due to heat of adsorption and therefore, anoncombustible adsorbent such as a molecular sieve is more preferablyused.

The temperature at the adsorption and removal of iodine is preferably aslow as possible because the adsorbed amount increases, however, acooling device or the like need not be installed so as to avoid acomplicated constitution of the removing device. The adsorptiontemperature is preferably 100° C. or less, more preferably 50° C. orless.

Iodine is a sublimable substance and, therefore, the iodine is partiallysolidified at a temperature of sublimation pressure or less. If thepassing of gas is continued in such a state, this may cause clogging ofthe pipeline. In this case, the clogging can be prevented by heating thepipeline connecting the decomposition reactor and the adsorption columnto a temperature of vapor pressure or more and thereby inhibiting theiodine-containing gas from dropping to a temperature of sublimationpressure or less.

Furthermore, iodine which is gasified in the high-temperature area maybe cooled and solidified at the adsorbent inlet to cause clogging andstop the passing of gas. This clogging by iodine can be prevented byheating the container packed with the adsorbent to a temperature of notcausing solidification of iodine. Also, in the case of dropping thetemperature to the vapor pressure or less of iodine because a lowertemperature is advantageous for the adsorption as described above, acoarse-meshed wire gauze or the like is deposited on the inlet portionhaving a decreased temperature so as to prevent the solidification atthe adsorbent inlet. By this treatment, iodine at higher than thesublimation pressure can be fixed at that portion and iodine of onlysublimation pressure can be passed to the adsorbent, as a result, iodinemore than the saturated vapor pressure cannot be passed to the adsorbentand therefore, the solidification at the adsorbent inlet can beprevented. The volume of the wire gauze may be appropriately determinedaccording to the amount of iodine produced by decomposition or theadsorption temperature. However, if the volume of wire gauze is toolarge, the amount of adsorbent decreases to cause an earlybreak-through, whereas if it is too small, clogging due to iodineoccurs. Therefore, the volume of wire gauze is preferably from 1/100 to½, more preferably from 1/10 to ¼, of the adsorbent volume. In addition,if the wire gauze packed is too fine, clogging disadvantageously occursdue to the iodine being fixed. Therefore, the roughness of wire gauzeused is, in terms of porosity, preferably 80% or more, more preferably90% or more. This porosity shows the percentage of portion except forthose used for fixing, such as wire gauze, occupying in a certainvolume. That is, the porosity is 100% when a gauze or the like is notpresent, whereas the porosity is 0% when all portions are filled withthe gauze.

When clogging occurs due to iodine, gas cannot pass through. Therefore,whether clogging is brought about by iodine must be monitored. This maybe monitored by a method of providing a manometer before the portionconsidered to clog and confirming whether the pressure is raised byclogging, or a method of providing a flow meter at the inlet and theoutlet and confirming whether the same linear velocity is maintained.

If the linear velocity into the adsorbent is too large, thisdisadvantageously incurs a problem that break-through is readily causedor pressure drop increases, whereas if it is too small, a large-sizeadsorption column is necessary and this is not preferred in view ofinstallation space or handling. Accordingly, the linear velocity ispreferably from 0.1 to 20 Nm/min, more preferably from 1 to 10 Nm/min.

As the adsorbent for removing iodine used in the present invention, acommercially available adsorbent can be used as described above. Forexample, Coconut Shell Activated Carbon (produced byAjinomono-Fine-Techno Co., Inc.) and Molecular Sieve 13X (produced byUnion Showa) can be preferably used. An adsorbent supplied from amanufacturer can be packed as it is into the adsorption column and used.In the case of using an adsorbent stored for a long period of time, thiscan be used as if it was brand-new by, for example, drying it at 150 to300° C. in an inert gas flow.

The shape of the adsorbent is not particularly limited and the removalby adsorption can be performed with any shape such as columnar form orspherical form. As for the size of adsorbent, if the particle size istoo large, the surface area participating in the adsorption anddiffusion of iodine becomes relatively small and the diffusion proceedsat a low rate. On the other hand, if the particle size is too small, thesurface area participating in the adsorption and diffusion of iodinebecomes relatively large and the diffusion proceeds at a high rate,however, as the amount of gas to be treated becomes large, thedifferential pressure also becomes large and this hinders theminiaturization or the like of the adsorption container. Accordingly,the particle size of adsorbent is preferably from 0.5 to 10 mm, morepreferably from 1 to 5 mm.

The reactive agent for decomposition used in the present inventioncomprises alumina and an alkaline earth metal compound and has afunction of decomposing a fluorine compound having iodine within themolecule and fixing the produced chlorine, fluorine and/or sulfur as analkaline earth metal salt.

The alumina used is not particularly limited but it is important toselect an appropriate starting material having few impurities. In thepresent invention, for example, an activated alumina or apseudo-boehmite alumina can be used as the alumina raw material. Apseudo-boehmite alumina is particularly preferred.

The content of alkali metals contained as impurities in the alumina ispreferably 0.1 mass % or less, more preferably 0.01 mass % or less,still more preferably 0.001 mass % or less.

Examples of the alkaline earth metal compound which can be used includea carbonate, a hydroxide and an oxide of alkaline earth metal. Amongthese, carbonates of magnesium, calcium, strontium and barium arepreferred, and a carbonate of calcium is more preferred. Similarly toalumina, the total content of alkali metals contained as impurities inthe alkaline earth metal compound is preferably 0.1 mass % or less, morepreferably 0.01 mass % or less, still more preferably 0.001 mass % orless.

The reactive agent for use in the present invention may further containat least one oxide of copper, tin, nickel, cobalt, chromium, molybdenum,tungsten and vanadium. Examples of the metal oxide include copper oxide(CuO), tin oxide (SnO₂), nickel oxide (NiO), cobalt oxide (CoO),chromium oxide (Cr₂O₃), molybdenum oxide (MoO₃), tungsten oxide (WO₂)and vanadium oxide (V₂O₅). Among these, copper oxide and tin oxide arepreferred. For example, in the case where copper oxide or tin oxide isused in the reactive agent and present together with alumina and analkaline earth metal compound, carbon monoxide produced by thedecomposition depending on the kind of fluorine compound can be oxidizedeven to carbon dioxide in a low oxygen partial pressure. Similarly tothe above-described raw materials, the total content of alkali metalscontained as impurities in this metal oxide is preferably 0.1 mass % orless, more preferably 0.01 mass % or less, still more preferably 0.001mass % or less.

In the reactive agent for use in the present invention, the contentratio of alumina and an alkaline earth metal compound added to thereactive agent is, in terms of mass ratio, preferably from 1:9 to 1:1,more preferably from 1:4 to 2:3. The alumina in the reactive agentefficiently decomposes a fluorine compound by coexisting with analkaline earth metal compound and assuming that the mass of the entirereactive agent is 1, the alumina content is preferably 0.1 or more bymass at least at the initial time of decomposition reaction, though thecontent may fluctuate as the decomposition reaction proceeds. If thisratio is less than 0.1, the fluorine compound may not be sufficientlydecomposed. However, if alumina is contained in an amount exceeding themass ratio of 0.5, this is accompanied with reduction in the amount ofalkaline earth metal compound and the effective utilization factor ofreactive agent decreases.

The content ratio of metal oxide is preferably, in terms of the massratio to the total amount of alumina and alkaline earth metal compound,from 1:99 to 5:95. If this ratio is too small, a sufficiently higheffect may not be obtained, whereas if it is excessively large, theeffect may be saturated, the total amount of alumina and alkaline earthmetal compound relatively decreases and the fluorine compound may not bedecomposed with good efficiency.

In granulating the reactive agent by blending alumina and an alkalineearth metal compound and further blending one or more oxide of copper,tin, nickel, cobalt, chromium, molybdenum, tungsten and vanadium, wateror, depending on the particle size of raw material, water and a binder,can be added. The binder is not particularly limited as long as it doesnot affect the blended raw materials. Assuming that the total mass ofraw materials blended is 1, the binder can be added in an amount of 0.03to 0.05 in terms of the mass ratio to the total mass. The binder ispreferably fine powder alumina. By adding fine powder alumina,respective raw materials are more improved in the dispersibility, and adifficulty in the granulation of an alkaline earth metal compound can beovercome. The particle size-of alumina added as the binder is suitably0.1 μm or less and the total content of alkali metals contained asimpurities is preferably 0.1 mass % or less, more preferably 0.01 wt %or less. However, as long as the binder does not affect the capabilityof the obtained reactive agent for decomposition, the kind and theamount of binder are not limited.

As described above, each of the raw materials blended in the reactiveagent, including the fine powder alumina added as the binder, preferablyhas a total alkali metal content of 0.1 mass % or less. Also, the totalalkali metal content in the reactive agent is preferably 0.1 mass % orless. If the total alkali metal content in the reactive agent exceeds0.1 mass %, the active sites on the alumina surface decrease and this isconsidered to sometimes cause reduction in the decomposition ratioparticularly of PFC gases such as CF₄ and C₂F₆.

In producing the granular reactive agent for use in the presentinvention, respective raw materials are blended and then kneaded whileadding an appropriate amount of water, and the kneaded product isgranulated to provide a granular article. The granular article is thendried at from 100 to 200° C. in an inert gas such as nitrogen or in airso as to evaporate water. The reasons why the reactive agent is used asa granular article are to enhance the decomposing activity of reactiveagent and to increase the hardness so as to prevent crushing or flouringduring filling into a reactor or handling. For this purpose, thegranular article is preferably further calcined. More specifically, thegranulated and dried article is calcined at from 400 to 700° C.,preferably from 500 to 700° C. in an inert gas such as nitrogen or inair. The reasons why the granular article is calcined at 400° C. or moreare to further evaporate water added during the granulation and therebyenhance the decomposing activity and to increase the hardness. If thecalcining temperature exceeds 700° C., the decomposing ratio (activity)of reactive agent sometimes decreases though it is not clearly knownwhether this is ascribable to the decomposition of alkaline earth metalcompound (for example, CaCO₃→CaO+CO₂). In other words, it is importantto almost completely remove the bound water of alumina (pseudo-boehmite)at 700° C. or less where the activity of reactive agent does notdecrease. The water content in the calcined reactive agent is preferablysuch that the amount of water content released on heating at 550° C. inan inert gas or air atmosphere is 1 mass % or less. As for the equipmentused for calcining, the calcining may be performed in a continuoussystem such as rotary kiln or in a stationary furnace.

As described above, the reactive agent for decomposing a fluorinecompound, which is used in the present invention, comprises alumina andan alkaline earth metal compound as essential components. Furthermore, ametal oxide such as CuO, SnO₂, NiO, CoO, Cr₂O₃, MoO₃, WO₂, and V₂O₅ mayalso be added to the reactive agent so that carbon monoxide produced bythe fluorine compound can be oxidized at a low oxygen partial pressure.The reactive agent is preferably in a granular form for increasingopportunities of contact with a fluorine compound to be decomposed. Ifthe particle size of reactive agent is too large, the surface areaparticipating in the adsorption and diffusion of fluorine compound gasbecomes relatively small and the diffusion proceeds at a low rate. Onthe other hand, if the particle size is too small, the surface areaparticipating in the adsorption and diffusion becomes relatively largeand the diffusion proceeds at a high rate, however, as the amount of gasto be treated increases, the differential pressure also becomes largeand this hinders the miniaturization or the like of reactor.Accordingly, the particle size of reactive agent is suitably from 0.5 to10 mm, preferably from 1 to 5 mm.

The process for decomposing a fluorine compound and rendering itharmless of the present invention is described below. Aniodine-containing fluorine compound having is contacted with a reactiveagent, produced by the above-described method, at an appropriatetemperature and, as a result, the fluorine compound is decomposed andchlorine, fluorine and/or sulfur produced by the decomposition are fixedon the reactive agent as a chloride, a fluoride or the like of alkalineearth metal. Furthermore, when, for example, a carbonate is used as thealkaline earth metal compound, carbon derived from the fluorine compoundis oxidized by oxygen released upon decomposition of the carbonate andis mostly released as CO₂ or CO. Here, iodine which is generated by thedecomposition and cannot be fixed can be removed by contacting it withan adsorbent. In this case, the reaction temperature varies depending onthe kind of compound contained in the untreated gas.

CF₃I, which is a fluorine compound having iodine within the molecule,thermally decomposes at 500 to 600° C. but when the above-describeddecomposition reactive agent is used, this compound decomposes at 200 to300° C. In the case where many kinds of PFC or HFC (hydrofluorocarbon)are used or contained as in the exhaust gas after etching or cleaning inthe production process of a semiconductor, the gases produced asby-products all must be also rendered harmless. For example, PFC whichis a by-product gas is classified as a difficult-to-decompose compoundof the fluorine compounds. Particularly, CF₄, C₂F₆ and the like are mostdifficult to decompose and for decomposing these gases only by thermaldecomposition, a high temperature of 1,200 to 1,400° C. is necessary.However, when the above-described reactive agent is used, these gasescan be decomposed at 500° C. or more. In this way, the decompositiontemperature varies over a fairly large range depending on the kind ofcompound. Therefore, it is important to set the reactor at an optimaltemperature according to the kind of compound.

As the reaction temperature varies depending on the kind or structure ofcompound, the reaction temperature is suitably set to 500° C. or more.Depending on the kind of compound, the treated gas (exhaust gas)sometimes contains CO. However, by allowing oxygen to coexist in the gasto be treated, CO can be easily oxidized into CO₂ and the gas can berendered completely harmless.

The fluorine compound concentration in the gas to be treated is notparticularly limited, however, an excessively low concentration isdisadvantageous in view of profitability. On the other hand, if theconcentration is too high, the reaction temperature elevates due to theheat generated by the decomposition, though this may vary depending onthe kind of compound. Therefore, in order to avoid occurrence of thecase where the temperature within the reactor can hardly be controlledor to prevent iodine from clogging the pipeline, the fluorine compoundconcentration is preferably from 0.01 to 10 vol %. The gas to be treatedis more preferably diluted with an inert gas or an oxygen-containing gas(including air) to have a fluorine compound concentration of 0.1 to 5vol %, still more preferably from 0.1 to 3 vol %. However, when thereaction temperature can be controlled by enforcedly removing the heatgenerated upon decomposition, the fluorine compound concentration is notlimited to the above-described range.

In this way, suitable reaction conditions are preferably establishedaccording to respective cases by taking account of the kind andconcentration of the fluorine compound in the gas subjected to thedecomposition treatment, the oxygen gas concentration in the gas to betreated, SV (space velocity), LV (linear velocity) and the mixed statewith other gases.

The decomposition treatment may be performed using a decompositionapparatus comprising a reactor filled with the above-described reactiveagent, an inlet for the gas to be treated, which is provided tocommunicate with the inside of the reactor, a gas outlet provided fordischarging the gas out of the reactor after the reaction, a furnace forhousing the reactor, and a heat source for elevating the furnaceatmosphere to a predetermined temperature, where the inlet for gas to betreated is connected with a fluorine compound gas source through apipeline.

FIG. 1 is a view showing one example of the apparatus for practicing theprocess of the present invention. While previously passing a constantamount of carrier gas from a nitrogen gas supply line 2 or from an airor oxygen gas supply line 3, a reactive agent 10 filled in a reactor 7is heated to a predetermined temperature by an electric heater 8 andcontrolled to a constant temperature by using a temperature sensor 12provided in the reactor 7 and a temperature controlling unit 9.

At the predetermined temperature, gases to be treated are introducedinto a mixing chamber 5 through respective valves from a supply line 1for the gas of a fluorine compound having an iodine atom within themolecule and from the nitrogen gas supply line 2 or air or oxygen supplyline 3. If desired, other gases are introduced into the mixing chamber 5from a supply line 4. Then, the mixed gas to be treated is introducedinto the reactor 7 through a gas inlet tube. The gas to be treated,which is introduced into the reactor 7, is contacted with the reactiveagent heated to a predetermined temperature and thereby decomposed. Thetreated gas (exhaust gas) after decomposition is introduced into anadsorption column 15 through a pipeline kept warm by a pipeline heatingheater 13. In the adsorption column 15, an iodine fixing layer 16 isprovided in the upper part and an adsorbent 17 is filled in thedownstream part thereof. Incidentally, for sampling a gas, samplingports 6 for analysis of reactor inlet gas, 14 for analysis of reactoroutlet gas and adsorption column inlet gas, and 18 for analysis ofadsorption column outlet gas may be provided, whereby the components ofeach gas can be analyzed.

In this way, the fluorine compound in the gas to be treated is almostcompletely decomposed and the iodine discharged from the reactor is alsoremoved by adsorption. The halogens such as fluorine and the carboncomponent in the decomposed fluorine compound react with the alkalineearth metal compound in the reactive agent, whereby, for example, thefluorine component is fixed on the reactive agent as a stable alkalineearth metal fluoride such as CaF₂, the carbon component is mostlydischarged as CO₂ together with the diluting gas such as nitrogen gas,and the iodine is fixed on the adsorbent. Accordingly, the treated gasbecomes a harmless gas substantially free of harmful materials such asfluorine component, iodine or carbon monoxide.

The decomposition reaction terminates when the decomposing ability ofreactive agent filled is exhausted. This end point of decompositionreaction is known by the time when the fluorine compound is firstdetected. The fluorine compound may be decomposed in a batch systemwhere, when the fluorine compound is detected and the reactive agentloses the decomposing ability, the operation of apparatus is stopped andafter newly filling the reactive agent, the decomposition reaction isre-started, or by a system where, in the same apparatus, the reactor issequentially exchanged with a spare reactor previously filled with thereactive agent.

The break-through of adsorbent can be known by monitoring the iodineconcentration at the adsorbent outlet. When break-through is reached,the adsorption column is exchanged with a spare iodine adsorption columnpreviously filled with the adsorbent, whereby the adsorption can beagain performed.

In order to continuously use the batch system, a multiple tower switchsystem may also be adopted, where a plurality of reactors and adsorptioncontainers of the same type are juxtaposed, and the reactive agent ofone reactor and/or the adsorbent of one adsorption container isexchanged while another reactor and another adsorption container areoperating, or a reactor previously filled with the reactive agent and/oran adsorption container previously filled with the adsorbent isexchanged and when one reactor and/or one adsorption column is stopped,the gas passage is switched to another reactor or adsorption column.

The present invention is further illustrated below by referring toExamples, however, the present invention should not be construed asbeing limited to these Examples.

Preparation of Reactive Agent

The substances shown in Table 1 below were used as the raw materials ofreactive agent. TABLE 1 Name of Raw Specific Material of ParticleSurface Area Impurities [mass %] Reactive Agent Size [μm] [m²/g] Na K FeSi CaCO₃ (high-purity 40 0.0012 0.0005 <0.0001 <0.0001 calciumcarbonate) Al₂O₃[AlO(OH)] 60 241 0.0027 <0.001 Fe₂O₃ SiO₂(pseudo-boehmite 0.0034 0.0066 alumina) CuO (Cupric oxide) 4 to 10 <0.01<0.01 <0.01 <0.01 SnO₂ (stannic oxide) 4 to 10 <0.01 <0.01 <0.01 <0.01Binder (ultrafine powder   <0.1 <0.001 <0.001 <0.001 <0.001 alumina)

The substances shown in Table 1 were used as raw materials and mixed ina Henschel mixer and, after adding water, the mixture was granulated andthen heated at 110° C. for 3 hours. The resulting granules were sievedto obtain a granular product having a particle size of 0.85 to 2.8 mm.The obtained granular product was dehydration calcined by a heattreatment at a calcining temperature of 550° C. for 3 hours in an airatmosphere to prepare a reactive agent.

Decomposition Treatment of Gas to be Treated

The process of the present invention was performed using an apparatushaving the same principles as the apparatus shown in FIG. 1. Namely,along the axial center of a cyclic furnace (electric capacity: 1.5 KW)with a heating element capable of generating heat on passing of anelectric current, an SUS reaction tube having an external form of ¾ inch(thickness: 1 mm) and a length of 50 cm was inserted and 25 ml of thereactive agent for decomposition was filled in the reactor. In theregion extending from the outlet of reaction tube to the inlet ofadsorption column, a ribbon heater was wound and heated to 60° C. so asto prevent fixing of iodine. As the adsorption column, an SUS adsorptioncolumn having an exterior form of ½ inch (thickness: 1 mm) and a lengthof 50 cm was used and at the inlet of adsorption column, 2 ml of an SUSpacking material (porosity: 95%) was packed and 10 ml of an adsorbentwas filled.

A fluorine compound gas having iodine within the molecule was used as agas to be decomposed and a test where a PFC gas was allowed to coexistwith the fluorine compound gas was also performed.

In either test, the gas to be treated was introduced after charging ofthe heating element was started, while controlling the quantity ofelectricity in the cyclic furnace so that the temperature measured by athermocouple inserted into the center part of the reactive agent (thesite reaching a highest temperature in the bulk of the reactive agent)could be maintained at a predetermined temperature. In Table 2 below,the decomposition reaction temperature indicates this temperaturemaintained during the reaction.

The gas to be treated and the treated gas were sampled from respectivesampling ports and the analysis results of each composition are shown inTable 2. In the analysis, O₂, N₂ and fluorine compound were analyzedusing a gas analyzer, and iodine was sampled into a detector tube and/oran absorption bottle containing potassium iodide and analyzed bytitration.

EXAMPLE 1

The decomposition reaction and removal by adsorption were performedunder the conditions shown in Table 2.

In the reactor outlet gas, the concentration of the objective gas oftreatment was less than the detection limit. Also, in the adsorptioncolumn outlet gas, iodine could be removed to less than the allowableconcentration.

EXAMPLE 2

The operation of Example 1 was repeated except for changing thedecomposition reaction temperature to 300° C. In the reactor outlet gas,the concentration of the objective gas of treatment was less than thedetection limit. Also, in the adsorption column outlet gas, iodine couldbe removed to less than the allowable concentration.

EXAMPLE 3

The operation of Example 2 was repeated except for changing theconcentration of the objective gas of treatment to 0.1 vol %. In thereactor outlet gas, the concentration of the objective gas of treatmentwas less than the detection limit. Also, in the adsorption column outletgas, iodine could be removed to less than the allowable concentration.

EXAMPLE 4

The decomposition reaction and removal by adsorption operation wereperformed by adding CF₄ as the objective gas of treatment under theconditions shown in Table 2. In the reactor outlet gas, theconcentration of the objective gas of treatment was less than thedetection limit even in the presence of perfluorocarbon. Also, in theadsorption column outlet gas, iodine could be removed to less than theallowable concentration.

EXAMPLE 5

The operation of Example 4 was repeated except for changing thedecomposition reaction temperature to 300° C. In the reactor outlet gas,CF₄ could be little decomposed. The concentration of CF₃I was less thanthe detection limit. In the adsorption column outlet gas, iodine couldbe removed to less than the allowable concentration.

EXAMPLE 6

The operation of Example 5 was repeated except for changing theadsorbent to activated carbon. In the reactor outlet gas, theconcentration of the objective gas of treatment was less than thedetection limit. Also, in the adsorption column outlet gas, iodine couldbe removed to less than the allowable concentration. TABLE 2 ReactorInlet Conditions and Treatment Conditions Ex- Kind of am- DilutingTreatment Results, Reactor Outlet and Adsorption Column Outlet pleObjective Gas of Treatment Gas 1 hr 2 hr 3 hr 5 hr 10 hr Results(reactor outlet gas) 1 CF₃I: 1.2 vol % O₂/N₂ = 3/97 CF₃I Decompositionratio, % >99.9 >99.9 >99.9 >99.9 >99.9 Decomposition reactiontemperature: 550° C. trace of CF₄ Adsorbent: MS-13X I₂ Concentration,vol % 0.54 0.57 0.63 0.58 0.59 Results (adsorption column outlet gas)Total flow rate: 190 Nml/min I₂ Concentration, vol ppm <0.1 <0.1 <0.1<0.1 <0.1 Results (reactor outlet gas) 2 CF₃I: 1.2 vol % O₂/N₂ = 3/97CF₃I Decomposition ratio, % >99.9 >99.9 >99.9 >99.9 >99.9 Decompositionreaction temperature: 300° C. trace of CF₄ Adsorbent: MS-13X I₂Concentration, vol % 0.51 0.52 0.52 0.49 0.51 Results (adsorption columnoutlet gas) Total flow rate: 190 Nml/min I₂ Concentration, vol ppm <0.1<0.1 <0.1 <0.1 <0.1 Results (reactor outlet gas) 3 CF₃I: 0.1 vol % O₂/N₂= 3/97 CF₃I Decomposition ratio, % >99.9 >99.9 >99.9 >99.9 >99.9Decomposition reaction temperature: 300° C. I₂ Concentration, vol %0.052 0.048 0.049 0.047 0.051 Results (adsorption column outlet gas)Adsorbent: MS-13X I₂ Concentration, vol ppm <0.1 <0.1 <0.1 <0.1 <0.1Total flow rate: 190 Nml/min Results (reactor outlet gas) 4 CF₃I: 0.5vol % O₂/N₂ = 3/97 CF₃I Decomposition ratio,% >99.9 >99.9 >99.9 >99.9 >99.9 CF₄: 0.5 vol % CF₄ Decomposition ratio,% >99.9 >99.9 >99.9 >99.9 >99.9 Decomposition reaction temperature: 550°C. I₂ Concentration, vol % 0.25 0.21 0.26 0.23 0.26 Results (adsorptioncolumn outlet gas) Adsorbent: MS-13X I₂ Concentration, vol ppm <0.1 <0.1<0.1 <0.1 <0.1 Results (reactor outlet gas) 5 CF₃I: 0.5 vol % O₂/N₂ =3/97 CF₃I Decomposition ratio, % >99.9 >99.9 >99.9 >99.9 >99.9 CF₄: 0.5vol % CF₄ Decomposition ratio, % 2 1 3 1 2 Decomposition reactiontemperature: 300° C. I₂ Concentration, vol % 0.25 0.22 0.26 0.21 0.22Results (adsorption column outlet gas) Adsorbent: MS-13X I₂Concentration, vol ppm <0.1 <0.1 <0.1 <0.1 <0.1 Total flow rate: 190Nml/min Results (reactor outlet gas) 6 CF₃I: 0.5 vol % O₂/N₂ = 3/97 CF₃IDecomposition ratio, % >99.9 >99.9 >99.9 >99.9 >99.9 CF₄: 0.5 vol % CF₄Decomposition ratio, % >99.9 >99.9 >99.9 >99.9 >99.9 Decompositionreaction temperature: 550° C. I₂ Concentration, vol % 0.22 0.23 0.270.24 0.26 Results (adsorption column outlet gas) Adsorbent: ActivatedCarbon Y-10 I₂ Concentration, vol ppm <0.1 <0.1 <0.1 <0.1 <0.1 Totalflow rate: 190 Nml/min

Industrial Applicability

As described in the foregoing pages, according to the present invention,a fluorine compound having an iodine atom within the molecule can bedecomposed, the components except for iodine can be rendered harmless,and iodine can be removed, whereby the decomposition treatment of a gasgenerated after a fluorine compound having an iodine atom within themolecule is used, particularly in the production of a semiconductordevice, is facilitated.

1. A process for decomposing a fluorine compound, comprising bringing agas containing a fluorine compound having iodine within the moleculeinto contact with a reactive agent containing alumina and an alkalineearth metal compound, and then bringing the gas obtained after saidcontact, into contact with an adsorbent.
 2. The process as claimed inclaim 1, wherein said alumina is pseudo-boehmite alumina.
 3. The processas claimed in claim 1 or 2, wherein said alkaline earth metal compoundis a carbonate of magnesium, calcium, strontium and/or barium.
 4. Theprocess as claimed in any one of claims 1 to 3, wherein said reactiveagent contains an oxide of at least one metal selected from the groupconsisting of copper, tin, nickel, cobalt, chromium, molybdenum,tungsten and vanadium.
 5. The process as claimed in any one of claims 1to 4, wherein the alkali metal content of said reactive agent is 0.1mass % or less.
 6. The process as claimed in any one of claims 1 to 5,wherein said reactive agent is a granule having a particle size of 0.5to 10 mm.
 7. The process as claimed in claim 6, wherein the watercontent of said reactive agent is 1 mass % or less.
 8. The process asclaimed in any one of claims 1 to 7, wherein the fluorine compoundhaving iodine within the molecule is contacted with said reactive agentat a temperature of 200° C. or more.
 9. The process as claimed in anyone of claims 1 to 8, wherein the gas containing a fluorine compoundhaving iodine within the molecule is an exhaust gas generated during useof said fluorine compound for etching or cleaning.
 10. The process asclaimed in any one of claims 1 to 9, wherein the gas containing afluorine compound having iodine within the molecule is contacted withsaid reactive agent at a temperature of 500° C. or more in the presenceof oxygen to thereby restrain the production of carbon monoxide.
 11. Theprocess as claimed in any one of claims 1 to 10, wherein said adsorbentis at least one adsorbent selected from the group consisting ofactivated carbon, alumina, silica gel and zeolite.
 12. The process asclaimed in any one of claims 1 to 11, wherein the gas is contacted withsaid adsorbent at a linear velocity of 20 Nm/min or less.
 13. Theprocess as claimed in any one of claims 1 to 12, wherein a layer forfixing iodine is provided in front of the layer packed with saidadsorbent.
 14. The process as claimed in claim 13, wherein the layer forfixing iodine has a porosity of 80% or more.
 15. The process as claimedin any one of claims 1 to 14, wherein the fluorine compound havingiodine within the molecule is at least one compound selected from thegroup consisting of fluoroiodocarbon, hydrofluoroiodocarbon,chlorofluoroiodocarbon and hydrochlorofluoroiodocarbon.